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The standard model for eruptive flares has in the past few years beenextended to 3D. It predicts typical J-shaped photospheric footprints of thecoronal current layer, forming at similar locations as the Quasi-SeparatrixLayers (QSLs). Such a morphology is also found for flare ribbons observed inthe EUV band, as well as in non-linear force-free field (NLFFF) magnetic fieldextrapolations and models. We study the evolution of the photospheric traces ofthe current density and flare ribbons, both obtained with the SDO instruments.We investigate the photospheric current evolution during the 6 September 2011X-class flare (SOL2011-09-06T22:20) from observational data of the magneticfield obtained with HMI. This evolution is compared with that of the flareribbons observed in the EUV filters of the AIA. We also compare the observedelectric current density and the flare ribbon morphology with that of the QSLscomputed from the flux rope insertion method/NLFFF model.  The NLFFF model shows the presence of a fan-spine configuration of overlyingfield lines, due to the presence of a parasitic polarity, embedding anelongated flux rope that appears in the observations as two parts of afilament. The QSLs, evolved via a magnetofrictional method, also show similarmorphology and evolution as both the current ribbons and the EUV flare ribbonsobtained at several times during the flare. For the first time, we propose acombined analysis of the photospheric traces of an eruptive flare, in a complextopology, with direct measurements of electric currents and QSLs fromobservational data and a magnetic field model. The results, obtained by twodifferent and independent approaches, 1) confirm previous results of currentincrease during the impulsive phase of the flare, 2) show how NLFFF models cancapture the essential physical signatures of flares even in a complex magneticfield topology.

Evolution of Flare Ribbons, Electric Currents and Quasi-separatrix Layers During an X-class Flare